Current-limiting circuit for cathode-ray tubes



f'L/RREDT THROUGH Nov. ll, 1952 J. BLAYNEY 517,964

'CURRENT-ummm@ CIRCUIT Foa cATHonE-RAY TUBES RES/5705*' .30

Nov. 1l, 1952 J. BLAYNEY 617,964

CURRENT-.LIM'ITING CIRCUIT FOR' CATHODE-RAY TUBES Filed Deo. 3o, 1949" a'srmETS--SHEET 2 Patented-Nov. 1l, 1952 CURRENT-LIMITING CIRCUIT FOR CATHODE-RAY TUBES Application December 30, 1949, Serial N0. 135,977

3 Claims.

The present .invention relates to television receivers, and more particularly relates to means for preventing damage to the image-reproducing cathode-ray tube which may result from a iiow of beam current in excess of that for which the tube is designed.

Television receivers commonly include a plurality of alternating-current amplifying stages. These would normally act to prevent low-frequency components in the composite television signal from being present at the control grid of the image-reproducing device, and hence changes in the average illumination of the scene being televised would not be faithfully reproduced at the receiver. This condition is overcome by the use of so-called "'D.C. reinserting circuits, which develop a bias potential for the cathoderay tube the value of which is determined by the average value of the video signal relative to a xed reference voltage level. It is also customary to employ, in addition to this circuit, a manually-adjustable background control, which permits changes to be made in the average brilliance of the reproduced image to suit the taste of different observers.

In order to simplify the operation of the television receiver, it is desirable that this manual background control be eliminated. rllhis is practicable in cases where the high-voltage supply for the cathode-ray tube is well regulated, and where the D.-C. restoration circuit is designed for efcient operation. Such an expedient, however, has heretofore subjected the cathode-ray tube to possible damage when the receiver contrast control is manipulated beyond the point where the beam current of the cathode-ray tube exceeds its maximum rated value. The very fact that the D.-C'. restoring circuit operates eiiiciently over a wide range of video signal voltages acts to maintain the cathode-ray tube cutoi point at a substantial coincidence with the Video signal blanking level over this entire range, and hence permits a strong video signal in the direction of white to cause an excessive scanning beam current to flow. Since the blanking level of the video signal under such conditions remains substantially at the cathode-ray tube cut-oil potential, the high-contrast image is not objectionable to many observers, and hence the abovedescribed conditions may continue for a considerable period of time. The life of the cathode-ray tube is thus appreciably lowered, and irreparable damage to its uorescent screen may result.

It is accordingly desirable toA provide means whereby a television receiver incorporatingan automatic background control circuit may operate so that the maximum beam current which can flow in the cathode-ray tube either is limited;

to a safe value, or else so that a flow of current in the tube which exceeds this value will cause such a degradation of picture quality that an observer will of his own volition reduce the gain of the receiver. I

In accordance with one feature of the invention, therefore, means are provided whereby a television receiver incorporating an automatic D.C. restoration circuit may have its contrast control Varied over the safe operating range of the the cathode-ray tube without affecting the normal D.C. restoring action. However, if an observer manipulates the contrast control in a direction which would tend to exceed such a safe operating value, then means are provided whereby a large amount of degeneration is introduced into the cathode circuit of the imagereproducing tube. The blanking level of the composite television signal is accordingly no longer maintained at the cut-01T voltage point of the cathode-ray tube, and the video portion of the signal is pushed into the black region. This eiectively reduces the tube beam current, and tends to prevent the harmful effects mentioned above. Furthermore, since such a mode of operation results in av poorly reproduced image, the observer is enticed to manipulate the contrast control so as to reduce the receiver gain to a point within the proper tube operating range. Thus an upper limitr is effectively established for the contrast control setting which coincides with the maximum permissible beam current in the cathode-ray tube.

One object of the present invention, therefore, is to provide a protective system for television receivers in which an effective limitation is placed upon the amount of cathode-ray tube current which is controllable by an observer.v

Another object of the present invention is to provide a television receiving system in which the operating bias for the cathode-ray image reproducing tube is supplied from a limiter tube functioning as a cathode follower, and in which use is made of the change in impedance of this cathode follower unit when sufficient cathoderay tube current is drawn so that the limite tube is rendered non-conductive.

Other objects and advantages will be apparent from the following description of preferred forms of the invention and from the drawings,

in which: v v

Fig. 1 is a schematic representation of a protective circuit in accordance With the present invention;

Figs. 2 and 3 are graphs illustrating the mode of operation of the circuit of Fig. 1;

Fig. 4 is a modification of the circuit of Fig. 1; and

Figs. 5' and' 6 illustrate impedances'e'quivalent to those present in the circuit of Fig. 4 under different operating conditions.

Referring first to Fig. 1, there is, showira portion of a television receiving system including a cathode-ray image-reproducing tube I0. Tube I0 is of conventional design, such', for example, as a BP4 or a TP400. Itis providedY with a cathode I2, a heater I4, a grid or control electrode I6, and an accelerating7 electrode` or sec,- ond anode I8. The remaining elements, of tube I0, such as the first anode and the deecting means, form no part of the present'J invention and hence have not been illustrated in order to `sim-- plifjy the drawing. It is assumedl that the heat'- ingelement I4 issupplied with suitable current andf that the second anode I8'- is connected to a suitable' sourceof positive operating; potential.

The television receiver of which tube II'! forms apart includes a series of electron discharge devices for amplifying the video portion of the received composite television signal. The final unit in this video amplierf chain comprises an output tube 20` which operates in a conventional manner to supply video signal informationv tothe control grid- IIi of the cathode-ray tube I0. The output of this video amplifier tube 250 (as developed across its load/resistor 22) isA of such nature that blanking pulses appear on thecontrol grid |16; with negative polarity, while image signals representative of white extend in a positive direction.

' In orderl automaticallyto control the average brilliancy; or^ background illumination, of the image reproducedby the tube IfII so that it may-correspond to the average brilliancyv of the image being` transmitted, a- D.-C. restoring circuit 24 isincludedA between the video amplifier tlube andthe-cathoderayrtube I0. This D.C. restoring'circuit,l or automatic background contrord'evice,` 24 `may beiof any suitable type known thefart which isfeiective to maintain on the control grid I6 'a bias potential the value of which istdetermined" by theaverage value of the videoA signaloutputof' tube 20relative to a fixed'reference voltage level. Inasmuch as the details ofi theqD.-C1 restoring circuit 24 form no partici: the vpresent invention, the unit hasbeen indi# cated inthe drawing by a labeled rectangle.

The cathode I2 of the image-reproducing tube I'UoA isv connectedi directly to the cathode 25 of a limiten tube V1. The latter mayY be a triod'e which is arranged as a cathode-follower unit, with its anode 28- being connected' directly to a source of positive operating potential' (such as -5350 volts-) and# with itsl cathode connected to ground through a varia-ble resistor 30'. The con.-

trollelectrode 32- or tube V1 is connected: to the. movable element of al potentiometer? 311;. which acts,- inamanner-to be later described, as arbiasi control; for the cathode ray tube I Theltvvo.

terminals of the resistance element? ofA potenti-- ometer 34 are 'respectively connected-.to the source of positive operatingspotential'for tube vV1 through an impedance 36'-, andi togiroun'd through. a further impedance 38'.v

Accordingly, the positive. potential applied` to the control'. electrode 3212 ofv tube V1 may be varied by an adjustment. off. poi-1 tentiometer 34, while the flow of current through the tube may be varied by adjustment of the resistor 30.

Cathode-ray tube I0 is designed to operate with a beam current that does not exceed a certain predetermined value. This upper limit on the beam current of tube It,VV is so chosen that the life of thetube shall bel at a maximum, and so that its iiuorescent screen shall not be damaged by the impact thereon of an electron stream of excessiveintensity. Although the maximum current ratings for cathode-ray tubes vary considerably in practice, a figure of 140 microamperes is representative for a tube of the TP400 type. Means will now' be'described whereby this maximum beam current cannot be exceeded without an indication `tothe observer that the television receiver is not operating at its maximum efciency;

The image-reproducing tube I0 has its cathode I2, connected directly to the. cathode 2BA of tuber V1. biased to cut-oifj in the absence. of a, video. sig*- nal of positive polarity applied to its: grid IE5v a from the ampli'er 20. If it be assumed that thev the potentiometer S4' is adjusted untilthe Voltage on the grid 32 is approximately the same as,` the potential required on cathode 2t in order to cut off the cathode-ray tube lil; or approximatelyv volts in the example. given; At the same time, the cathode resistor is adjusted until the current flow through tube V1 is equal approximately to the maximum allowable beam current of tubeV I0 (approximately 140 microamperes in the present example). conditions now existing, tube I0 is= cut. oil,A tube V1 is passing a current of` approximately 140 microamperes, and its cathode 26 has a; potential of approximately 125 volts positive. It is of course assumed that no video signal is being applied Y to the'CRT control grid I'. The relationship of' current iiow accordingly will be as set forth in Fig. 2, with all of the current through the resistorn 30 also flowing throughthe limiting tube V1, and no-part of it owing in the cathode-ray tube I0. Itl will be appreciated from an inspection of Fig; 1 that the impedance which is seen from th'ecathode I2 of tube I0 looking toward the limiter tube assembly is that of thecathoderesistor 3!! shunted bythe relatively lower impedance of the limiter tube itself. While the.Y adjusted value ofk the cathode resistor 30 may be. in the order of oneV megohm, forexample, the; impedance; of the triode V1 is equal to the reciprocal ofstheftube gm. In a tube having a high transconductance. (at

3.0. Furthermore, the current through tubeV V1 depends almost entirely upon the. potential ofiits control electrode 32, as determined by they setting of' potentiometer 34', andv also uponthe' adjustment of, resistor 30, being virtually independent of., tube characteristics.

Thus. a. condition existsiwhere. all of the cur-- rent which passes through the cathode resistor tiltfflows:through` the tube: V1. and'noi beam cur- It is desired that` tubelil be. normally` Under the rent flows in the cathode-ray tube I0. Assume now that a video signal of positive polarity is applied to the control electrode I6 of the cathoderay tube so that current flows in the tube and an image raster is traced on the uorescent screen thereof. As will be seen from Fig. 1, this CRT beam current must flowy through the resistor 30. However, the amount of current which flows through this resistor depends upon the potential drop thereacross, and this turn is determined by the potential of the cathode 26, since the other end of the resistor is grounded. This cathode potential, during conduction of tube V1, can vary only slightly from the potential of the control electrode 32, and the latter is held constant at the adjusted potential of the potentiometer 34. Hence, the flow of cathode-ray tube current through resistor 30 cannot bring about a substantial change in the potential of cathode 26, and the total current through the cathode resistor 36 remains substantially constant. The result is that the flow of current through the limiting tube V1 decreases in proportion to the increase in cathode-ray tube current, as shown in Fig. 2. It has been found in practice (using a circuit designed along the lines of Fig. l) that with an initial cathode-ray tube bias of +125 volts, and with an initial current of 140 microamperes flowing through the resistor 30, an increase in video signal voltage on the control electrode I6 of the cathode-ray tube I0 results in a division of cathode-ray tube current and limiting tube current as set forth in the graph of Fig. 2. The point at which the limiting tube current reaches zero (or in other words the point at which the current flowing through cathode resistor 30 equals the beam current in the cathode-ray tube i0) represents the maximum permissible video signal voltage which may be applied to the cathode-ray tube I6 from the video amplifier 26 without exceeding the safe operating limit for which the cathode-ray tube is designed Under the conditions set forth, the bias voltage on the cathode-ray tube cathode I2 remains substantially constant (actually it increases by only 4 volts) while the cathode-ray tube beam current increases from zero to a value of approximately 140 microamperes. At this point, the current flowing through the resistor 30 equals the cathode-ray tube current, and the limiter tube V1 is biased to cut-off.

It has been mentioned above that the impedance seen from the cathode I2 of tube I6 during the time that the limiter tube V1 is conducting comprises the fairly high resistance 30 shunted by the relatively low resistance of the tube V1. When the point is reached at which V1 is cut 01T. however, the low shunting impedance of the latter tube is no longer present, and the impedance seen by the cathode I2 consists solely of the resistor 36. A further increase in current flow through this resistor as the receiver contrast contol is advanced will cause an increase in anode current flow in the cathode-ray tube I0, but this increase will be at a rate such as shown in Fig. 3. That is, from the point of cut-oir of tube V1 the curve of beam current bends sharply, so that at the maximum setting of the video gain control (representing an increase in the CRT bias from +129 to approximately +170 volts) the anode current of the CRT has reached a value of only 190 microamperes. This is comparable to a beam current of approximately 400 microamperes (a linear rise) which is usual in conventional television receivers.

Fig. 4 shows a modification of the circuit of Fig. 1 in which the resistor 30 of the latter arrangement is replaced by an electron discharge tube. As shown in Fig. 4, the cathode 26 of tube V1, instead of being connected to ground through the resistor 30 as in Fig. '1, is joined to the anode 40 of a further triode V2. The control electrode 42 of this triode V2 is grounded, while its cathode 44 is connected to one terminal of an adjustable resistor 46 the other terminal of which is grounded through a fixed resistor 48. The tubes V1 and V2 may, if desired, constitute the two sections of a 12AU7.

If theseries-conneoted resistors 46 and 48 of Fig. 4 be designated as the cathode resistance Rk of the triode V2, then the equivalent circuit of Fig. 4 will be as shown in Figs. 5 and 6 under varying conditions of operation. For example, when the triode V1 is conducting (or, in other words, when the beam current of the cathode-ray tube I0 is less than approximately 140 microamperes in the example given) then the impedance seen by the cathode I2 of the cathoderay tube consists of a series of impedances TD2, R11 and itzRk, all of these series impedances being shunted by the impedance Since the latter is equal substantially to of tube V1, it is of relatively low value, and the voltage on the cathode 26 remains substantially constant.

As soon as tube V1 is biased to cut off, however, the shunting impedance lll is removed, and the equivalent circuit of Fig. 6 applies. The three impedances m1 R12, and 2R12 are now present in series between the cathode I2 of tube I0 and ground. Any increase in current flow through these series-connected impedances results in a substantial increase in bias voltage on the cathode I2, with the result that the video signal is pushed into the CRT cut-oir region.

The increased effectiveness of the circuit of Fig. 4 over that of the circuit of Fig. 1 is brought about primarily by a replacement of the static impedance of resistor 30 in Fig. 1 by the dynamic impedance of the triode V2 in Fig. 4, the latter tending to exhibit a greater resistance to a change in the amount of current flowing therethrough.

The potentiometer 34 is provided for the purpose of permitting a selection of the initial bias voltage which is placed upon the cathode I2 of the image-reproducing tube I0. Once this bias voltage is properly selected, it need not normally be changed under ordinary operating conditions.

The following values are given as illustrative of those which have been found suitable for use in the circuits of Figs. 1 and 4. However, they are to be considered as merely exemplary, and are not to be construed in a limiting sense:

Resistor 48, 5600 ohms 7, InFlgs; and 6 the valuesv of the various impedances` may be as follows:

Erg 100 k.

fiz-Rk1-2 megohms Having, thus described my invention, I claim: 1. In combination, a cathode-ray tube having a cathode source of electrons, means for producing a beam of said electrons, and an element adapted t0 be supplied with a video signal to control the intensity of said beam; a vacuum tube having atleast triode elements; a source of anode potential for said vacuum tube; av nonlinear. impedance having one terminal conductively connected to the cathode of both of said tubes and having its other terminal conductively connected` to a point of xed potential, said nonlinear im',- lpedance having the characteristic lof incr-easing in impedance with increasing current therethrough; and means for varying the bias of the grid of said vacuum tube relative to said point of fixed potential to vary the current in said nonlinear impedance and thereby to vary the potential of the cathode of said cathode-ray tube to control the average beamk current thereof, whereby, when said vacuum tube is biased to conduct, it is operative to reduce the eiective impedance between the cathode of said cathode-ray tube and said point of xed potential.

2. In combination, a cathode-ray tube having a cathode source of electrons, means for producing a beam of said electr-ons, and an element adapted to be supplied with a video signal to control the intensity of sai-d beam; a irst vacuum tube having at least triode elements; a source oi anode potential for said first vacuum tube; a nonlinear impedance having one terminal conductively connected to the cathodes of both of said tubes and having its other terminal conductively connected to a point of iiXed potential, said nonlinear impedance comprising a second vacuum tube having at least triode elements, said second Vacuum tube having its anode connected to the cathode of said first vacuum tube, its grid connected to a point of fixed potential and its cathode connected through an impedance to said inst-mentioned point of xed potential; and means' for varying the bias of the grid of said irst vacuum tube relative to said first-mentioned point of xed potential to vary the current in said nonlinear impedance and thereby to vary the potential of the cathode of said cathoderay tube to control the average beam current thereof', whereby, when said first vacuum tube is biased' to conduct, it is operative to reduce the eiective impedancev between the cathode oi.'- said cathode-ray tube and said point of fixed potential.

3; In combination, a cathode-ray tube having a cathode source of electrons, means for producing a beam of said electrons, and an element adapted to be supplied with a video signal to control the intensity of said beam; a first vacuum' tube having at least triode elements; a source of anode potential for said rst vacuum tube; a nonlinear impedance having one terminal conductively connected to the cathodes of both said tubes and having its other terminal conductively connected to a point of xed potential, said nonlinear impedance comprising a second vacuum tube having at least triode elements, `said sec'- ond vacuum tube having its anode connected to the cathode of said rst vacuum tube, itsr grid connected to a point of fixed potential and its cathode connected through' a resistor t0 said first-mentioned point of xed potential, and means for biasing said grid of said rst Vacuum tube to a selected potential, the magnitude of said grid biasV potential being selected to cause said cathode of said cathode-ray tube to be at the desired operating potential when said rst vacuum tube is biased to conduct, the resistance of said resistor being selectedto permit a current now through said nonlinear impedance substan` tially equal to the maximum safe operating beam current of said'cathode-ray tube when said rst vacuum tube is biased to conduct, whereby the cathode of said cathode-ray tube is held at a substantially constant potential for values of beam current below said maximum safe operating value, and whereby the potential of said cathode oi said cathode-ray tube increases rapidly in response to increases in beam current beyond said maximum safe operating value, thereby limiting the rate of rise of` said beam current in response toincreases in said video signal.

JAMES LEONARD BLAYNEY.

REFERENCES CITED The following references are of record iny the' file of this patent:

UNITED STATES PATENTS Number Name Date 2,092,373 Holmes Sept. 7, 1937 2,178,764 Poch Nov. 7, 1939 2,222,943 George Nov. 26, 1940 2,227,603 Poch Jan. 7, 1941 2,255,484 Dome Sept. 9, 1941 2,261,787 Wendt Nov. 4, 1941 2,280,670, Spielman Apr. 21, 1942 2,371,897 Knick Mar. 20, 1945 2,465,364 Ferrar Mar. 29, 1949 2,520,155 Liekens Aug. 29, 1950 

